With a rise in the price of crude oil in recent years, the developments of oil and natural gas wells at greater depths and in more corrosive environments have been underway. For drilling for oil and natural gas in such harsh environments, oil country tubular goods have been required to have high-strength and an excellent corrosion resistance.
Oil and natural gas contain corrosive substances such as carbon dioxide (CO2), hydrogen sulfide (H2S), and chloride ions (Cl−). For this reason, materials for oil country tubular goods used for drilling for oil or natural gas are required to have an excellent corrosion resistance against these corrosive substances. Especially under environments being at high temperatures of 150° C. or more and including hydrogen sulfide at 1 atm or more, a main cause of corroding oil country tubular goods is stress corrosion cracking. Therefore, materials for oil country tubular goods used in such environments are required to have an excellent stress corrosion cracking resistances.
Furthermore, the growing demand of oil and natural gas has brought oil and gas wells at greater depths and higher temperatures. Materials for oil country tubular goods used in such wells are required to withstand a corrosion environment containing carbon dioxide, hydrogen sulfide and chloride ions, and to have high-strength at the same time. For this reason, in recent years, there has been a greater demand for high-strength oil country tubular goods having a high yield strength (0.2% offset yield stress) of 965 MPa or more.
The use of Ni—Cr alloy materials such as those disclosed in Patent Document 1 to 3 have been attempted in the above harsh corrosive environments, where low alloyed steels, such as martensitic stainless steels, or duplex stainless steels cannot be applied.
For example, in the alloy for oil country tubular goods in Patent Document 1, contents of Ni and Cr are adjusted within specific ranges, and a value of Cr (%)+10Mo (%)+5W (%) and a value of Mo (%)+(½)W (%) are adjusted within specific ranges. Patent Document 1 discloses that the above alloy can ensure the stress corrosion cracking resistance in a “H2S—CO2—Cl−” environment as long as the temperature of the environment is 150° C. or less even if it is a cold worked material. In addition, Patent Document 1 discloses that it is possible to achieve a 0.2% yield stress of 965 MPa or more by adjusting N concentration within a range of 0.05 to 0.30% by mass %, and performing cold working after solution treatment.
In the alloy for oil country tubular goods in Patent Document 2, contents of Ni and Cr are adjusted within specific ranges, and a value of Cr (%)+10Mo (%)+5W (%) and a value of Mo (%)+(½)W (%) are adjusted within specific ranges. Patent Document 2 discloses that the above alloy can ensure the stress corrosion cracking resistance in a “H2S—CO2—Cl−” environment having an extremely excellent corrosion property, in particular in an adverse environment at 150° C. or less, even if it is a cold worked material. In addition, Patent Document 2 discloses that it is possible to achieve a 0.2% yield stress of 965 MPa or more by making N contained within a range of 0.05 to 0.25% by mass %, and performing both cold working and aging treatment after solution treatment.
In the alloy for oil country tubular goods in Patent Document 3, contents of Mn, Ni, and Cr are adjusted within specific ranges, and a value of (½) Mn (%)+Ni (%), a value of Cr (%)+Mo (%)+(½)W (%), and a value of Mo (%)+(½)W (%) are adjusted within specific ranges. Patent Document 3 discloses that the above alloy can ensure the stress corrosion cracking resistance in a “H2S—CO2—Cl−” environment having an extremely strong corrosive properties, in particular, an adverse environment at 150° C. or less, even if it is a cold worked material. In addition, Patent Document 3 discloses that it is possible to achieve a 0.2% yield stress of about 940 MPa by making N contained within a range of 0.1 to 0.4% by mass %, and performing cold working after solution treatment.
Note that, strengthening by increasing the N content as in Patent Document 1 to 3 described above is prone to cause a problem of reducing hot workability of the alloy. For this reason, Patent Document 1 to 3 disclose a technique to improve hot workability by reducing a content of S to 0.0007% or less by mass %, or by making Ca, Mg, Ti, or the rare-earth metal elements (hereafter, referred to as “REM”) contained.
In the high Cr-high Ni alloy material in Patent Document 4, contents of Cu, Ni, and Cr are adjusted within specific ranges, and a value of Cu+0.4(Mo-1.4)2 is adjusted within a specific range. Patent Document 4 discloses that the above alloy can ensure the good stress corrosion cracking resistance in a corrosive “H2S—CO2—Cl−” environment even when it is subjected to 25% cold working to bring the 0.2% yield stress of the alloy to a strength level of 861 to 964 MPa (87.75 to 98.28 kgf/mm2), so-called “125 ksi-class”.
Note that Patent Document 4 also discloses a technique to improve the hot workability by reducing a content of S to 0.0007% or less by mass %, or by making Ca, Mg, REM contained.
In addition, Patent Document 5 discloses a super austenitic stainless steel having an excellent corrosion resistance in an acid environment and a seawater environment, and it is also excellent in the hot workability by adjusting contents of Cr, Ni, Mo, Mn, and N within specific ranges and making elements such as Mg, Ca, and Ce contained.
In the Cr—Ni alloy material in Patent Document 6, contents of Cu, Ni, Cr, Mo, N, Al, and REM are adjusted within specific ranges, and a value of N (%)×P (%)/REM (%) is adjusted within a specific range. Patent Document 6 discloses that the above alloy material can ensure the good hot workability, and ensure the good stress corrosion cracking resistance in a corrosive environment including H2S, Cl−, and the like at a temperature of 177° C. by performing cold rolling of 40% reduction of area to provide a high 0.2% yield stress of 941 to 1176 MPa.
Patent Document 7 discloses a producing method for a stainless steel in which contents of Cr, Ni, Si, Mn, C, N, Mo, S, B, P, and O are adjusted within specific ranges. Patent Document 7 describes that the above stainless steel is excellent in strength and the stress corrosion cracking resistance.
Patent Document 8 discloses an austenitic alloy in which contents of C, Si, Mn, Cr, Ni, Mo, La, Al, Ca, O, P, and S are adjusted within specific ranges. Patent Document 8 describes that the above austenitic alloy has a good crack resistance in an environment including hydrogen sulfide.